1. Field of the Inventions
The present inventions relate to a lubricating apparatus for an internal combustion engine comprising a feed pump and scavenging pumps.
2. Description of Background Art
In a wetsump type lubricating apparatus, lubricating oil accumulated in an oil pan arranged at a lower part of an internal combustion engine is directly fed to parts required to be lubricated in the engine. In some engines, a drysump type lubricating system is used in place of the wetsump type apparatus.
In a drysump type apparatus, an oil reservoir is provided in addition to an oil pan for collecting the lubricating oil from the oil pan. The lubricating oil accumulated in the oil reservoir is fed to an oil tank by scavenging pump(s). The lubricating oil can remain in the oil tank for a time. Then the lubricating oil is fed by a feed pump from the oil tank to parts of engine required to be lubricated.
In a vehicle such as an all terrain vehicle designed for traveling off-road or on a steeply sloped road, it is desirable that the vehicle has a sufficient ground clearance for off-road use, as well as a low center of gravity. Accordingly, an oil pan, such as that commonly used in wetsump systems, is undesirable because the large undulations on the bottom surface of such an oil pan interferes with ground clearance and/or requires the engine to be mounted higher, thereby raising the center of gravity. If a drysump type system is used, it remains difficult to configure the oil reservoir to operate properly even when the body of vehicle is inclined.
One example of a prior art lubricating apparatus for an internal combustion engine which can help to alleviate these problems is shown in FIGS. 7 and 8. As shown in FIG. 7, an oil pump unit 50 comprises a feed pump 51, a main scavenging pump 52 and a sub-scavenging pump 53 and is arranged between a front case 60a and a rear case 60b of a crank case 60. Rotors 54, 55 and 56 of these pumps 51, 52 and 53 are mounted on a common driving shaft 63 so as to be driven integrally therewith.
Oil reservoirs 57 and 58 are arranged at the bottom of the crank case 60 and are separated in a fore and aft direction by a partition wall 59. The main scavenging pump 52 and the sub-scavenging pump 53 separately draw oil from the oil reservoirs 57 and 58, respectively. The oil reservoir 57 is arranged substantially at a middle of the bottom of the crank case 60 which is a position where most of the oil tends to collect when the vehicle body is in a horizontal position. On the other hand, the oil reservoir 58 is arranged at a position shifted rearwardly relative to the reservoir 57 and extends in the right and left directions so that the oil can easily move even when the vehicle body is inclined toward the fore and aft directions or the right and left directions.
As shown in FIG. 8, a relief valve 61 is arranged between a passage of the discharge side of the feed pump 51 and a passage of the discharge side of the main scavenging pump 52 and the sub-scavenging pump 53. As such, the relief valve 61 can open and relieve surplus oil when a pressure in the passage of the discharge side of the feed pump 51 exceeds a predetermined relief pressure. The surplus oil is then fed to an oil tank 64 via an oil cooler 62 together with oil discharged from the main scavenging pump 52 and the sub-scavenging pump 53.
This structure allows the main scavenging pump 52 and the sub-scavenging pump 53 to reliably draw the oil from the reservoirs 57, 58. Additionally, this arrangement provides more freedom in the design of the bottom portion of the crank case 60 and thus it is easier to lower the center of gravity of vehicle while keeping a sufficient ground clearance relative to the crank case 60 (see e.g. Japanese Laid-open Patent Publication No. 73731/2001).
Recently, however, it has become more desirable to further lower the center of gravity in vehicles such as all-terrain vehicles and water-vehicles, while maintaining sufficient ground clearance relative to the crank case. In the lubricating apparatus of the prior art noted above in which three pumps 51, 52 and 53 are arranged within the crank case 60 of engine, the capacity of the oil reservoir and the oil tank is reduced due to the pumps and partition walls. This problem is more severe in multi-cylinder engines, in that it is more difficult to draw oil from each of the oil reservoirs 57 and 58 in a stable manner while the position of vehicle changes, due to additional limitations in layout in the multi-cylinder engine.